1. Field of the Invention
The invention relates to circuits for controlling silicon controlled rectifiers, or the like.
2. Prior Art
Silicon controlled rectifiers (SCRs), and like devices have been employed for a number of years in rectifiers, latching switches, amplifiers, and other applications. One of the most common uses for such devices is for the rectification of alternating current (AC) power to provide direct current (DC) power. Numerous control circuits are known for providing the firing pulses which initiate the conduction in the SCRs. These circuits include the half-wave R-C-diode firing circuit, on-off magnetic firing circuit, phase control firing circuits, reversing type control circuits, and numerous other circuits.
Special circuits have been developed for controlling SCRs in larger power applications, particularly where multi-phase AC power is rectified. One such firing circuit generates ramp signals, which signals are compared with a loop (analog) error voltage or with a fixed reference voltage. The results of this comparison are used to generate firing signals or pulses. The ramp generators used in this circuit are triggered at the AC power signals, by way of example, when these signals pass through zero volts. Thus the firing pulses occur at a fixed time following the "zero-crossing" of the AC power signal.
In some instances the AC power signal becomes very distorted. This distortion may include the introduction of harmonics into the power waveform, changes of frequency, and other distortions. Typical firing circuits rely on the AC power signal as a reference signal and do not operate properly when the AC power signal is distorted or of varying frequency. For example, the zero-crossing of the AC signal may be substantially shifted when harmonics are introduced into the signal. Also where ramp signals are employed with comparators, a change in frequency of the power signal causes a change in the electrical angle, but not the time at which the firing pulses occur. Unless the SCRs are precisely fired, i.e. ideally firing at a fixed electrical angle, inefficient rectification occurs. In some cases, SCRs are misfired due to distortion in the power waveform, causing a malfunction in the rectifier and introducing spurious disturbance into associated control and regulation loops.
Distortion of the AC power signal may be caused by the generation means, by the load, by the transmission means used to couple the generation means with the load, or some combination of these elements. In one specific application the output of a high inertia, three-phase alternator is rectified and used to drive a large electromagnet. The magnet is energized for short periods (e.g. 1 to 10 seconds) during which time power (with peaks in the megawatt range) is transferred from the alternator to the magnet. (The electromagnet is used in plasma experiments). Because of the inductive nature of this load, substantial distortion occurs in the AC power signal. Moreover, the fundamental frequency of this signal changes, that is, the rate of rotation of the alternator slows when the load is applied. The firing circuit of the present invention performs well in this application. It will be obvious to one skilled in the art that the inventive concepts disclosed in this patent may be used in numerous other applications.